Small 'helper' stars needed for massive star formation

Feb 28, 2008

In order for a rare, massive star to form inside an interstellar cloud of gas and dust, small "helper" stars about the size of the sun must first set the stage, according to a new theory proposed by astrophysicists at the University of California, Berkeley, and Princeton University.

Massive stars between 10 and 150 times the mass of the sun are few in number but produce the bulk of the heavy elements in a galaxy when they explode in supernovas. Their extreme brightness makes them signposts of star formation in distant galaxies.

Astrophysicist Christopher F. McKee, professor of physics and astronomy at UC Berkeley, and Mark R. Krumholz, a Hubble postdoctoral fellow in the Department of Astrophysical Sciences at Princeton, have been modeling the formation of these stars for nearly 10 years. Recently, they looked at the conditions inside cold clouds of molecular hydrogen that favor formation of massive stars over low-mass stars like the sun.

In a report this week in Nature, Krumholz and McKee argue that early formation of a few low-mass stars in a cloud paves the way for later formation of a stellar big brother instead of fragmentation of the cloud into a hundred smaller clouds, which would produce only low-mass siblings.

"It's only the formation of these low-mass stars that heats up the cloud enough to cut off the fragmentation," McKee said. "It is as if the cold molecular cloud starts on the process of making low-mass stars but then, because of heating, that fragmentation is stopped and the rest of the gas goes into one large star."

"What it comes down to is that if a cloud is cold, it tends to break up into many small pieces that become low-mass stars," added Krumholz, who recently accepted a faculty position with the astronomy department at UC Santa Cruz. "As the cloud gets warmer, though, it can make bigger and bigger objects."

The cloud temperatures are still cold, however. A typical interstellar hydrogen cloud is 10-20 degrees Celsius above absolute zero (10-20 Kelvin, or about -430 degrees Fahrenheit), while low-mass stars can heat the cloud to double or triple this temperature. To stop the entire cloud from collapsing, the temperature would have to increase to many hundreds of degrees above absolute zero, McKee said.

According to Krumholz, each small star within a hydrogen cloud has a zone of influence where it warms up the gas and prevents it from collapsing into small fragments. In low density clouds, each zone of influence is small and contains very little mass, so this effect is unimportant.

As the density increases, however, the gas and small stars get packed closer and closer together. Eventually, said Krumholz, the zones of influence of the few low-mass stars encompass the entire cloud, preventing the cloud from fragmenting and forcing it to collapse to make a massive star.

McKee noted that this collapse occurs within an even larger interstellar cloud that may contain more than a million times the mass of the sun. Therefore, as in our galaxy's Orion Nebula, many massive stars may be forming simultaneously inside a giant molecular cloud.

The density above which massive stars can form is about a million hydrogen molecules per cubic centimeter, which is a very extreme vacuum on Earth, he said, but nevertheless dense enough to collapse into a massive star over hundreds of thousands of years. The particle density in Earth's atmosphere is 10 trillion times greater.

According to McKee, one implication of the density limitation is that in the outer reaches of galaxies, where the density may not reach this threshold in a sufficiently large region of space, low-mass stars may be forming in the absence of any massive stars. Because we can see only the big, bright stars from Earth, he said, astronomers may be underestimating the amount of star formation going on in distant galaxies.

"In fact, there may be many stars forming in the outer reaches of distant galaxies, just not the bright ones we can see," McKee said. "Star formation could be occurring that is essentially invisible."

He noted that a recent satellite collecting ultraviolet light from distant galaxies has seen evidence of star formation in the very outer regions of galaxies, and that this may confirm their prediction.

McKee and Krumholz are involved in large-scale computer simulations of star formation inside cold molecular clouds to confirm the researchers' mathematical theory that low-mass star formation is necessary for formation of high-mass stars.

This video simulates the collapse of a 100 solar mass protostellar core to a massive star. On the left is the entire molecular cloud about 1.3 light years on a side. On the right is a portion of that cloud magnified 40 times to focus on the formation of a single massive star. Without a few small stars to heat up the gas on the right, dozens of small stars would form instead of one massive star. (29.1 Mb .mpg)

Related Stories

Tracking the rotation speed of solid planets, like the Earth and Mars, is a relatively simple task: Just measure the time it takes for a surface feature to roll into view again. But giant gas planets Jupiter ...

Using data from orbiting observatories, including NASA's Spitzer Space Telescope, and ground-based facilities, an international team of astronomers has discovered an outburst from a star thought to be in ...

It's almost a rite of passage in physics and astronomy. Scientists spend years scrounging up money to build a fantastic new instrument. Then, when the long-awaited device finally approaches completion, the ...

A new Rice University-led survey of one of the most active star-forming regions in the galactic neighborhood is helping astronomers better understand the processes that may have contributed to the formation ...

A team of scientists led by astronomers at the University of Bonn discovered an unusual phenomenon in the centre of the Milky Way: They detected about 20 rotating dust and gas discs in each cluster hosting ...

Recommended for you

Designed to detect the fossil radiation of the universe, the Planck satellite, working in tandem with Herschel, can also help to understand the macrostructure of the universe. A just-published experimental ...

A scene of jagged fiery peaks, turbulent magma-like clouds and fiercely hot bursts of bright light. Although this may be reminiscent of a raging fire or the heart of a volcano, it actually shows a cold cosmic ...

By combining observations of the distant Universe made with ESA's Herschel andPlanck space observatories, cosmologists have discovered what could be the precursors of the vast clusters of galaxies that we ...

Stars form when gravity pulls together material within giant clouds of gas and dust. But gravity isn't the only force at work. Both turbulence and magnetic fields battle gravity, either by stirring things ...

Luke Skywalker's home in "Star Wars" is the desert planet Tatooine, with twin sunsets because it orbits two stars. So far, only uninhabitable gas-giant planets have been identified circling such binary stars, ...